The Oscar-nominated actress wore a bespoke Prada gown to the intimate event, rather than Versace as previously reported.
A source told us that the eight bridesmaids attending Mulligan also wore custom Prada dresses. The blue knee-length designs featured different necklines.
The choice of Prada echoes Mulligan's past red-carpet choices. She wore a Prada dress to the 2010 Oscars and often looks to Miu Miu for high-profile events.
Mulligan and Mumford, who decided to marry in August 2011 after a five-month courtship, were joined by guests including Sienna Miller, Jake Gyllenhaal, Dianna Agron and Colin Firth, who is said to have delivered a speech.
The ELLE cover star and Mumford met through church programmes as children, reconnecting at a Mumford & Sons gig in Nashville last February.